1. Field of the Invention
The present invention relates to a method for producing a powder-form toner for forming a visible image in electrophotography, electrostatic recording, and other imaging techniques, and to a positively chargeable non-magnetic single component toner containing no magnetic carrier particles, which can be employed for forming the visible image through being positively charged. Unless otherwise specified, the term “charged” refers to “electrically charged.”
2. Background Art
In an image formation apparatus employing an imaging method such as electrophotography or electrostatic imaging, a toner which has been charged in a predetermined amount and with a predetermined polarity is supplied to an image forming portion, and the toner particles are arranged imagewise on a recording sheet through application of an electric field. Through fixing the thus-arranged toner particles on the recording sheet, an image of interest is formed on the sheet.
The aforementioned toner employed in the image formation apparatus is generally formed of at least a binder resin (hereinafter referred to simply as a binder), a colorant, and a charge-controlling agent. The binder, which is a predominant component of the toner, is made of a thermoplastic synthetic resin. The binder is melted in the course of fixation of the toner, whereby the toner image is fixed on a recording sheet. The colorant, which is a substance for imparting a predetermined color to the toner, is generally a pigment or a dye. The charge-controlling agent is a substance for controlling chargeability of the toner.
Currently, two formats are widely known to supply a charged toner to an image forming portion; i.e., a magnetic two-component format and a non-magnetic single component format. The magnetic two-component format employs triboelectricity generated from magnetic carrier particles and a toner. The non-magnetic single component format employs no magnetic carrier particles, but triboelectricity generated from a toner and other members (e.g., a metal or synthetic resin plate-like blade and/or a roller made of synthetic resin). In recent years, the non-magnetic single component format has been more widely employed, since the format is advantageously employable in a small-scale image formation apparatus.
In order to consistently provide an excellent visible image of a toner, toner particles must be consistently charged. In other words, the toner particles must be charged as uniformly as possible, while the particles maintain a sufficient amount of electricity. In the magnetic two-component format, the magnetic carrier particles and the toner are almost uniformly mixed at a predetermined ratio, and therefore the toner particles are readily charged sufficiently and uniformly. From the viewpoint of production of sufficiently and uniformly charged toner particles, the magnetic two-component format employing the magnetic carrier is more advantageous than the non-magnetic single component format.
Therefore, when the non-magnetic single component format is employed, controlling the charging characteristics of toner particles by use of a charge-controlling agent is an important factor for providing sufficiently and uniformly charged toner particles.
Japanese Patent Application Laid-Open (kokai) No. 5-119513 discloses a toner formed of toner particles containing therein a charge-controlling resin (i.e., a synthetic resin serving as the charge-controlling agent) and containing the charge-controlling agent on the surfaces of the particles.
Japanese Patent Application Laid-Open (kokai) No. 10-293419 discloses a toner formed of generally spherical toner particles which surfaces are coated firmly with a thermoplastic resin region where a charge-controlling agent is uniformly dispersed. The toner is produced through the following procedure. Firstly, a synthetic resin which becomes self-dispersible or soluble in water through neutralization is neutralized with a neutralizing agent, to thereby prepare a self-water-dispersible or water soluble resin. Secondly, the self-water-dispersible or water soluble resin is mixed with a charge-controlling agent and an organic solvent, and the mixture is emulsified through phase inversion, to thereby form an aqueous dispersion having a particle size smaller than that of the generally spherical particles. Thirdly, the aqueous dispersion is mixed with a dispersion of the generally spherical particles. To the resultant mixture, another neutralizing agent having a polarity opposite that of the aforementioned neutralizing agent is added, followed by drying. In other words, the synthetic resin containing the aforementioned charge-controlling agent is fixed on the surfaces of the generally spherical particles by use of the second neutralizing agent.
The aforementioned toners have already been commercialized both in the positively chargeable type and the negatively chargeable type. Particularly in the aforementioned electrophotographic image formation apparatus, a positively chargeable toner is suitably employed so as to prevent generation of ozone.
The binder serving as a predominant component of the toner is generally formed of a polyester resin or a styrene-acrylic copolymer. However, these resins per se are prone to be negatively charged.
Thus, when a conventional polyester resin or styrene-acrylic copolymer is employed as a binder of a positively chargeable non-magnetic single component toner, the toner particles are difficult to charge sufficiently and uniformly.